Couple of thoughts.
Mine used to like bottles at a certain temperature - otherwise they wouldn't bother with them. Is it a new teacher? have they switched up how they give it to him?
I agree about hole in nipple. They have different sizes as they get older.
I'm not sure about the mixing cereal with the milk in bottle. We never did that.
I also offered food separate from the milk. I didn't give bottles (or breastfeed) with the meals. I liked to make sure they had empty belly before food (not full up on milk) and likewise, I didn't breastfeed right after a meal. I sort of spaced it out.
Try giving him a bottle at home (same kind you have at the daycare). Just to see how it goes. That might give you a better indication of what's going on (just 1/4 bottle so you can still breastfeed afterwards).
I can't remember what the actual amounts are they need at that stage (you can go over that with your pediatrician) but sounds like he's still having wet diapers so that's a good sign.
Good luck :)